Chiral Catalysts and Catalysis
The present invention relates to the field of asymmetric catalysis. More particularly, the invention relates to the field of organometallic catalysts useful for enantioselectively epoxidizing prochiral olefins.
Several advances in catalysis of asymmetric group transfer have occurred in recent years. One such advance has been the discovery by K. B. Sharpless et al. of the epoxidation of allylic alcohols which provides access to enantiomerically pure synthetic building blocks. Unfortunately, Sharpless catalysis requires the presence of a specific functional group, namely an allylic alcohol, on the olefin to be epoxidized. Naturally, this requirement severely limits the variety of olefins which can be so epoxidized.
Some success has been achieved in asymmetric catalysis of unfunctionalized olefins. For example, K. B. Sharpless reported in 1988 that certain cinchona alkaloid derivatives were effective ligands in the osmium-catalyzed asymmetric dihydroxylation of trans-stilbene and various other olefins. This method provides a practical route to certain chiral diols, although cis olefins afford poor results.
Aside from the catalysts disclosed herein, it is believed that there currently exists no practical catalytic method for the asymmetric epoxidation of unfunctionalized olefins. Some progress has been made in this area through the use of chiral porphyrin complexes. In particular, J. T. Groves et al. reported in 1983 the asymmetric epoxidation of styrene by a chiral iron porphyrin catalyst. Unfortunately, the Groves system suffers several disadvantages, namely, the porphyrin catalyst is relatively difficult to prepare, oxidant proceeds to low substrate conversion, is limited to styrene derivatives, and achieves enantiomeric excess (ee) values of less than about 50 percent.